Fuel cells have been developed as alternative power sources for motor vehicles, such as electrical vehicles. A fuel cell is a demand-type power system in which the fuel cell operates in response to the load imposed across the fuel cell. Typically, a liquid hydrogen containing fuel, for example, gasoline, methanol, diesel, naphtha, etc. serves as a fuel supply for the fuel cell after the fuel has been converted into a gaseous stream containing hydrogen. The conversion to the gaseous stream is usually accomplished by passing the fuel through a fuel reformer to convert the liquid fuel to a hydrogen gas stream that usually contains other gases such as carbon monoxide, carbon dioxide, methane, water vapor, oxygen, and unburned fuel. The hydrogen is then used by the fuel cell as a fuel in the generation of electricity for the vehicle.
A polymer electrolyte membrane type of fuel cell is generally composed of a stack of unit cells comprising a polymer electrolyte membrane enclosed between electrodes and gas diffusion layers, and further enclosed between separators that contain channels for fuel gas and oxidant gas. The stack is fixed by end plates. A current collector may be provided between the end plate and stack, or the end plate itself may function as current collector. When hydrogen is used as the fuel gas and oxygen is used as the oxidant gas, electrons are released due to a chemical reaction, and water is formed as a by-product, via the reaction:H2+½O2→H2O.
Consequently, the fuel cell is an energy source that has no adverse impact on the global environment, and has been the focus of much research for use in automobiles in recent years.
Although the water that is the product of the reaction is environmentally benign, when a large quantity of water reaction product accumulates in the fuel cell, it blocks the gas channels and the gas diffusion layers, causing a drop in electrical generation efficiency. In addition, if the fuel cell is exposed to temperatures at and below 0° C., which are common in the temperate and polar latitudes, the accumulated water freezes in the fuel cell and blocks the gas channels. It is not possible to generate electricity from frozen fuel cells when the gas channels are clogged with ice. Even if a heater is used to melt the ice, it takes time to melt the ice and thus rapid start up of an electrical vehicle is not possible with a frozen fuel cell.
Therefore, methods have been tried to reduce the quantity of water in the fuel cell stack before the stack freezes, to facilitate the generation of electricity in subfreezing ambients. One such method is to increase the flow rate of fuel gas and oxidant gas to blow the accumulated water out of the channels after electrical generation is shut down. Another method is to vacuum dry the channels. However, these methods take a substantial amount of time to dry the fuel cell to an acceptable level. This is probably because it is difficult to eliminate water from the small cavities in the gas diffusion layer and the electrode when attempting to dry the fuel cell by gas purge or vacuum drying.
When a fuel cell stack generates electricity at low temperature, the temperature does not rise uniformly throughout the stack. Along the stacking direction, the temperature in the central portion of the stack will tend to rise more rapidly and higher than portions of the fuel cell stack adjacent the end of the stack because the central portion is more distant from the ambient air. Conversely, the temperature at the end of the fuel cell stack is slow to rise, because it is nearer to the ambient air. As a result, the water produced when electricity is generated at the end of the fuel cell stack has been known to condense or freeze inside the stack, hindering the continued generation of electricity from fuel cells adjacent the end of the stack.
Time-consuming methods of drying or heating fuel cells in automotive applications are not feasible, because rapid startup is required and the fuel cell frequently cycles between operating and shut down states.